Seam testing method



Nov. 15, 1966 c. J. MATTIVI 3,285,056

SEAM TESTING METHOD Filed Nov. 12, 1963 INVENTOR Clifford J. Maffivi BY M1, w

AGENT United States Patent 3,285,056 SEAM TESTING METHOD Cliiford John Mattivi, Pittsburg, Kans., assignor, by mesne assignments, to Gulf Oil Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 12, 1963, Ser. No. 322,791 1 Claim. (Cl. 73-37) This invention relates to the testing of seams in sheet plastics. More particularly it relates to the inspection and testing of the heat-sealed seams in the ends of plastic bags, especially bags made from extruded plastic tubing and having no side seams.

In the manufacture of bags from plastic sheeting the side seams are often lapped, one edge over another, for sealing. A lap seam made by heat-sealing polyethylene sheeting, for example, is usually quite capable of withstanding the shearing stresses to which it is subjected. End seams, however, are usually made by pinching the walls of the bag together and heating to the softening point by means of a heated bar. These seams are subjected to tensile stress, concentrated along a line (similar to What is called T peeling) and are much more subject to failure than lap seams or seamless walls. In the manufacture of bags with end seams of this type it is necessary to inspect bags periodically to make sure that the bond between the two plastic surfaces is sufliciently strong. In the past this has necessitated cutting a sample out of the bottom of each bag which is selected for testing. This, of course, wastes every bag which is tested.

It has now been discovered that by using controlled internal pressure to subject a bag seam to a specified stress for a period of time, the strength of the bond can be measured by the extent to which the seam has opened under stress, and the usefulness of the bag will remain unimpaired. If a seam has insufficient strength it will fail, in which event it can be re-sealed and re-tested, but the bag will not be wasted.

Briefly, this is accomplished by means of the following steps:

(a) Inserting in the mouth of the bag a gas inlet,

(b) Temporarily closing the mouth of the bag sufliciently so that internal gas pressure can be exerted on the bag through the gas inlet,

(c) Con-fining the sidewalls of the bag within an enveloping structure so that the internal gas pressure is able to exert a deforming force against the end of the bag containing the seam, while limiting sidewall expansion of the bag,

(d) Subjecting the bag to a controlled, measured internal pressure for a period of time, and

(e) Measuring the width of the portion of the seam which has opened under the force exerted by the gas pressure.

In FIGURE 1 there is illustrated a simple apparatus which may be used to test heat-sealed bottom seams according to the method outlined above. FIGURE 2 is a view of a tapered plug of resilient material bearing a gas inlet and a connection for a pressure gauge which is shown in FIGURE I inserted in the mouth of a bag.

So that the invention may be more easily understood, the following description of specific embodiments thereof is presented by way of illustration but not limitation of the disclosed invention.

In typical operation of the test method with the appa- 3,285,056 Patented Nov. 15, 1966 ice ratus shown in FIG. 1, the tapered resilient plug, 1, shown in FIGURE 2 is placed in the mouth of a plastic bag, which is inserted in the enveloping enclosure 2. By pressing on the pedal, 3, the clamping jaws, 4, are brought together on the mouth of the bag containing the tapered plug, 1, producing a sufiiciently good seal that a gas, such as compressed air may then be admitted through the gas inlet, 5. As the gas is admitted to the bag, the pressure is monitored by means of pressure gauge, 6, attached to :gauge outlet, 7. A known controlled pressure is then maintained in the bag for a predetermined period of time, after which the pressure is released and the clamping jaws 4 are allowed to open. Normally, releasing the clamping jaws will cause force of escaping gas to move the bag backward, assisting in removal of the bag from the open rear end of the enveloping enclosure 2.

The stressing of the heat-sealed bottom seam for a specified period of time causes peeling of a portion of the width of the seam, plainly visible to the naked eye. By determining the proportion of the total width which has failed, a measure is obtained of the ability of seams in filled bags to withstand dropping and piling in stacks. In a representative test, a bag is kept under an internal pressure of 4.0 p.s.i.g. for one minute. The pressure is then released with the foot pedal and the bottom seal of the bag is inspected. If no more than 50 percent of the width of the seal has failed, the bag is considered to be acceptable. The bag which has been subjected to testing can be put to its normal use, it acceptable. If it is not acceptable, it can be resealed to make it acceptable, and can then be put to use.

Several variations in the apparatus will occur to those who are skilled in the art. For instance, the mechanical pinch clamp which holds the mouth of the bag shut can be replaced by a pair of inflatable tubes placed on opposite sides of the mouth of the bag. Such a clamping method has advantages in that clamping pressure is easily regulated and operation can be very quiet and rapid. The outlet 7, for attachment of a pressure gauge may be easily eliminated, if desired. The pressure gauge may instead be connected to a bladder or flexible tube placed inside the enveloping enclosure in contact with the inflated plastic bag. It is not necessary that the testing of the bag be done in the horizontal position. If it is necessary to 'be economical with floor space, a very compact apparatus which holds the bag in the vertical position may be easily constructed. In the vertical apparatus it is advantageous to place the bag mouth down and shape the tapered resilient plug 1 so that it extends upward about two-thirds of the length of the bag to serve as a guide in placing the bag in position. Other variations are possible within the scope of the invention as herein disclosed and claimed.

I claim:

A method of testing the strength of the bond in the heat-sealed bottom seam of a plastic bag comprising the steps:

(a) inserting in the mouth of the bag a gas inlet,

(b) temporarily closing the mouth of the bag sufii ciently so that internal gas pressure can be exerted on the bag through the gas inlet,

(c) confining the sidewalls of the bag within an enveloping structure so that the internal gas pressure is able to exert a deforming force against the end of the bag containing the seam, while limiting sidewall expansion of the bag, (d) subjecting the bag to a controlled, specific measured internal pressure for a'specified period of time, and (e) measuring the width of the portion of the seam which has opened under the force exerted by internal gas pressure.

References Cited by the Examiner UNITED STATES PATENTS 2,696,732 12/1954- Brant et a1. 73-37 2,821,851 2/1958 Daley 7337 4 FOREIGN PATENTS 6/1947 Norway.

References Cited by the Applicant RICHARD c. QUEISSER, Primary Examiner.

G. M. GRON, Assistant Examiner. 

